Conventionally, Automatic Gain Control (AGC) has widely been employed in order to converge a level of a received signal at a desired level in a radio reception apparatus. FIG. 5 is a schematic block diagram showing, as an exemplary conventional radio reception apparatus employing AGC, a configuration of a radio reception apparatus for use in a mobile communication system (a base station or mobile station thereof), for example, PHS (Personal Handyphone System).
Referring to FIG. 5, a burst signal in each frame of PHS received at an antenna 1 is amplified by a reception amplifier 2 with a variable gain. At the start of reception of a burst signal of each frame, the gain of reception amplifier 2 is preset at a certain initial value.
The received signal amplified by reception amplifier 2 is converted into a digital signal by an A/D converter 3 and is then subjected to quadrature modulation in a prescribed modulation method (for example, π/4QPSK (Quadrature Phase Shift Keying) method) by a quadrature modulator 4.
The output of quadrature modulator 4 is provided to a digital signal processing unit 5 for prescribed signal processing (for example, a synchronization process, propagation path estimation, weight estimation for adaptive arrays, and the like) and is also provided to an AGC control unit 6.
AGC control unit 6 monitors a digital output supplied from quadrature modulator 4, generates a control output for adjusting a variable gain of reception amplifier 2 such that a power level of the analog received signal in the frame converges at a prescribed level, and provides the control output to a gain control input of reception amplifier 2.
In this manner, in the conventional AGC operation, a burst signal in each frame of a received signal is received while its reception power level information is measured for use in adjusting the gain of the reception amplifier. For each frame, it usually takes approximately a few μ seconds from the starting point of time of each frame to converge an analog received signal level at a prescribed level through the above-described AGC operation.
Therefore, a few leading symbols of a digital signal for each frame obtained by performing A/D conversion on the output from reception amplifier 2 using A/D converter 3 during such a time period of about a few μ seconds may suffer erroneous amplitude values.
FIG. 6 is a diagram showing a waveform of a digital signal in a frame obtained through the above-described AGC operation. In FIG. 6, the axis of abscissas shows time (the number of symbols in the frame) and the axis of ordinates shows the magnitude (amplitude value) of the digital signal subjected to the AGC process.
As can be seen from the waveform diagram of FIG. 6, for example, if an initial value of a gain of reception amplifier 2 is set at a properly large value, a digital signal obtained in the beginning of the frame has a swell in its amplitude value and it takes a period of a few symbols to converge the amplitude value at a proper amplitude value through the AGC process. It is noted that it is actually impossible to set an appropriate initial value as an initial gain of reception amplifier 2 since the degree of fading varies greatly depending on a propagation path.
In a signal format of PHS, for example, a head portion of each frame includes a known signal segment including a preamble (PR), a unique word (UW), and the like. A known signal in this segment is used to perform a variety of signal processing as described above.
In the example shown in FIG. 6, a digital signal having an erroneous amplitude value occurs in a part of this known signal segment. Such a signal, however, is included as it is in the conventional digital signal processing segment. In other words, in the conventional mobile communication system such as PHS, since the well-known π/4QPSK method, for example, is generally employed as a modulation method, no reception error takes place during signal processing even if the amplitude value of the digital signal resulting from AGC is erroneous as described above.
More specifically, in the π/4QPSK method, a symbol point of a received signal corresponds to any of eight signal points positioned concentrically on an IQ coordinate plane, as is well known. Therefore, there is a one-to-one correspondence between a phase angle (direction) on the IQ coordinate plane and the symbol point. In other words, in this method, since a determination is made only based on a phase component of a digital signal, a symbol point is correctly recognized even if the amplitude value of the digital signal is inappropriate. Therefore, a reception error does not take place in signal processing at a subsequent stage.
Recent mobile communication systems, however, require data transmission of larger volume at higher speed, as compared with conventional voice communication. Accordingly, multi-value modulation methods having number of multi-values larger than the π/4QPSK method mentioned above have been developed. As an example of such multi-value modulation method, the well-known 16QAM (Quadrature Amplitude Modulation) method has been practically utilized in some type of data communication. According to the 16QAM modulation method, a symbol point of a received signal corresponds to any of a total of 16 signal points on the coordinate plane, arranged four by four in a lattice form in each quadrant of the IQ coordinate plane, as is well known. In other words, in this method, a determination of a symbol point is made based on both of a phase component and an amplitude component of a digital signal.
When the 16 QAM method is employed as a modulation method for PHS and if the amplitude value of the digital signal is inappropriate as shown in FIG. 6, a certain symbol point may possibly be recognized erroneously as another symbol point with the same phase with a different amplitude value, resulting in a reception error in signal processing at a subsequent stage.
The 16QAM method has already been put into practical use in some types of data communication. In a data format in such data communication, a sufficiently long signal segment is provided as a known signal segment such as a preamble. Even if the amplitude value takes an erroneous value resulting from AGC in a head portion of data, by ignoring the signal segment suffering this erroneous amplitude value, required signal processing can be performed sufficiently in the remaining known signal segment.
In the mobile communication system such as PHS, however, the known signal segment such as PR, UW is limited to a short segment of a few symbols at the head of each frame according to a signal format standard in order to secure transmission data volume. Therefore, when a multi-value modulation method such as the 16QAM method is employed, the amplitude value is erroneous in a large segment within the known signal segment, as shown in FIG. 6, resulting in a reception error during the digital signal processing.
An object of the present invention is therefore to provide a radio reception apparatus, a signal processing timing control method, and a signal processing timing control program, in which reception errors resulting from a AGC process are prevented even when a multi-value modulation method is employed to allow for high quality and large volume transmission, in a mobile communication system having a signal format that does not allow for a sufficient length of a known signal segment.